error[E0658]: declaration of a non-standard `#[main]` function may change over time, for now a top-level `fn main()` is required
  --> $DIR/feature-gate-main.rs:2:1
   |
LL | fn foo() {}
   | ^^^^^^^^^^^
   |
   = note: see issue #29634 <https://github.com/rust-lang/rust/issues/29634> for more information
   = help: add `#![feature(main)]` to the crate attributes to enable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0658`.